The present invention relates to a process for producing a chain or acyclic dimer of isoprene from the catalytic dimerization thereof.
More specifically, the present invention relates to a process for the catalytic production of 2,6-dimethyl-1,3,6-octatriene (hereinafter referred to as DMOT in some cases) wherein the catalyst used is unique.
Heretofore, as a catalyst for use in the production of the chain dimer of isoprene, DMOT, from the catalytic dimerization of isoprene, various types of Ziegler catalysts are known. Among these, a catalyst comprising a specific zirconium compound as one catalytic component has been proposed as a catalyst having a high selectivity of chain dimer formation.
Ordinarily, the catalytic dimerization reaction of isoprene is carried out in a solution. When this reaction is to be carried out on an industrial scale, a desirable embodiment is one which uses a larger proportion of isoprene feed with a smaller proportion of a solvent in order to improve productivity per reaction vessel.
However, the conventional zirconium-based catalysts have required a high concentration of catalyst and a high reaction temperature in order to increase the conversion of isoprene. This requirement of a high concentration of catalyst causes not only deterioration of the unit consumption of the catalyst (the zirconium-based catalysts being expensive), but also complication of the post treatment process after the reaction has been completed. That is, when the catalyst after the reaction completion is decomposed to remove it, the formation of an emulsion is accelerated, which makes it difficult to remove the catalyst. Furthermore, when the reaction is carried out at a high concentration of catalyst and a high reaction temperature, the conventinal catalyst systems often produce sticky polymer byproducts in the course of the reaction which stain the reaction vessel, rendering the continuation of the reaction infeasible. Accordingly, the development of a catalyst having a high activity at lower concentrations of the catalyst and lower reaction temperatures has been an important problem yet to be solved. As one approach to this problem, we have previously proposed a zirconium-based catalyst modified with an electron donor containing phosphorus, as disclosed in Japanese Patent Publication NO. 7565/75, and a catalyst system containing a zirconium compound having a special ligand, as disclosed in Japanese Patent Laid-Open Publication No. 50305/75.
In addition, we have made attempts to improve a titanium-based catalyst which is more catalytically active and less expensive than the zirconium-based catalysts but has a poor selectivity of chain formation and, as a result, have proposed a titanium-based catalyst modified with an iodine compound, as disclosed in Japanese Patent Application NO. 98872/74. With these types of catalyst systems which use an iodine compound as one of their features, the selectivity of chain dimer formation has been highly improved while maintaining the high activity characteristic of the titanium-based catalysts. Accordingly, the function and effect of the iodine compound can be said to be remarkably unique.
However, these catalysts are still not fully satisfactory in that the formation of cyclic dimer byproducts, the boiling point of which is close to that of the chain dimer, cannot be fully suppressed. As a result, the production of a chain dimer with high purity requires a relatively complex separation and purification process.